When Push Comes To Shove

Just had 4 fun-filled days (and 3 nights) at my sister's house. [You can't tell from the font, but that statement should be dripping with sarcasm.] She lives about 90 minutes away, and needed me to babysit my nieces Tuesday afternoon and Thursday evening. I had planned on going home in between, but my DH made plans for Wednesday night, the one night I was going to be home (I usually just sleep over at my sister's to avoid the 3-hour round-trip all in one day.) So I just set up shop at my sister's for the week!
I love my sister (all 3 of them actually) and I love my nieces (and nephews.) But I do not love spending 4 days trying to protect my 3- and 1-year-old nieces from my menace of a child. I don't know what got into her. Push push push. She spent practically every waking moment pushing one of her cousins. I want that toy! Push. I want that chair! Push. I'm just randomly walking by! Push. You're not bothering me in any way, so I'll go out of my way to push you! Push. You didn't fall over! Push again. In between pushing, she jumped on the couch (forbidden,) jumped OFF the couch (onto a pile of pillows,) ate 3x her body mass [or so it seemed,] and watched DVDs and Netflix. However, much of the show-watching was interrupted and punctuated by pushing.
To give you an idea of the extent of the problem: we arrived Tuesday, around 1pm. By 4pm, the 20-month old had 2 new words: "Push" and "Pushing." And she used them appropriately!
While babysitting I couldn't take Z for time outs with me (normally I remove her from the situation and sit with her for time-outs.) But placing her in a chair by herself seemed ineffectual. Placing her in the kitchen (a baby gate separates the family room from the kitchen) didn't deter. She enjoyed playing with the fridge magnets, and the 3-year-old kept wanting to be in the kitchen with her tormentor friend. Several times I had to just shut her into the baby's room so I could take care of my nieces without her causing more damage. (Like changing a poopy diaper, only to discover that I don't know where the diapers are stored. The normal stash in the living room had been depleted. So I had Z shut in the baby's room, an unhelpful 3-year-old and Free-Range-Baby-Goes-Commando in the living room while I searched the bedrooms and basement for clean diapers. I finally scrounged one out of our diaper bag, and then found Piwu playing on an entire package of them in the living room. Whuuuuuuck? I have no idea where she found them!)
Nothing worked.
She was surprisingly obedient in many cases - when I'd order her to sit at the kitchen table (so she wasn't playing with magnets and couldn't be seen by my niece) she'd go. She just wouldn't comply to stop behaviors, such as couch jumping and pushing. She'd stand there and ask if she could come back and play without pushing. And immediately walk over and push someone. She seemed to think pushing was a fun game, and was almost compulsive about it, and couldn't stop! I'm almost scared to take her to preschool on Monday.

I tried yelling. I tried calm and serious. I tried logic (although telling her that if she hurts her friends that they won't want to play with her loses its edge when said friend is begging and crying to sit back next to her, or to be put in the same room as the her.) I tried separation from the pushees, with and without me. (Without me, she just found stuff to play with. With me was less exciting, as I held her in my lap and read my book.) We tried rewarding the 'good' ones with activities (but by the time we got the paints out, say, they'd made up and Z had already been invited to partake in the fun by Tually.) Only thing left I can think of is spankings, but how will hitting her teach her that hitting is wrong? ["We don't solve our problems with violence!" *Smack!* Yeah.... Not so much.]
Plus she thinks spankings are fun.  -_-    [Remind me to write about that sometime!]